Project Summary/Abstract Traumatic brain injury (TBI) is the leading cause of injury-related death in patients under the age of 46 years. Survivors of the initial brain injury face numerous extracranial complications that are a major determinant of long-term outcome and impede their most productive years of life. Post-TBI pulmonary infection rate is as high as 50-60% and results in an infection-related mortality rate of ~30%. Additionally, after severe TBI, the parasympathetic nervous system is activated to attenuate the systemic inflammatory response and may prevent some post-TBI, non-brain end-organ injury. However, parasympathetic nervous system activation may be maladaptive and result in dysregulation of alveolar macrophage and neutrophil immune function within the lung. Our preliminary data reveal increased mortality after post-TBI pneumonia that is ?7 nicotinic acetylcholine receptor (?7nAChR)-dependent. ?7nAChR activation inhibits Pseudomonas aeruginosa-induced release of tumor necrosis factor-alpha (TNF-?) and nuclear factor-?? (NF- ??) activation in alveolar macrophages. Additionally, ?7nAChR activation inhibits phagocytosis of P. aeruginosa by alveolar macrophages. Interestingly, female patients have a lower incidence of post-TBI lung infection compared to males with equivalent TBI severity. Female mice have lower mortality and decreased lung bacterial burden after post-TBI pneumonia compared to males. Oophorectomized females have a post-TBI P. aeruginosa-induced pneumonia mortality rate similar to males and males that receive estrogen after TBI and just before onset of bacterial pneumonia have improved survival. Finally, estrogen inhibits the effects of ?7nAChR activation and restores appropriate TNF-? release, NF- ?? activation and phagocytic function. Thus, the question of how estrogen alleviates the ?7nAChR-dependent maladaptive response and improves survival from secondary pneumonia in TBI patients is of great clinical importance. Based on our preliminary data, we use a reverse translational approach to hypothesize that parasympathetic-induced, ?7nAChR-mediated dysregulation of alveolar macrophage and neutrophil immune function leads to decreased lung bacterial clearance and survival in post-TBI P. aeruginosa-induced lung infection and is alleviated by estrogen. To test this hypothesis, we propose three specific aims: (Aim 1) ?7nAChR activation leads to a dysregulated, maladaptive innate immune response leading to decreased lung bacterial clearance and survival after post-TBI P. aeruginosa infection; (Aim 2) Sex differences in parasympathetic-induced ?7nAChR maladaptive signaling leading to lung innate immune dysfunction are alleviated by estrogen; (Aim 3) Examine the sex differences in response to ?7nAChR activation of innate immune cells after TBI in humans. These findings will elucidate maladaptive ?7nAChR signaling and its molecular mechanisms leading to decreased lung innate immune function in post- TBI bacterial pneumonia and may provide a novel therapeutic strategy to reverse these effects and improve post-TBI patient mortality and long-term outcomes during their most productive years of life.